488 



ACCOUNT OF 



insure perfect accuracy in all instruments indicating 

 this change, because the pressure of a column of air 

 of a given altitude varies according to its humidity 

 or moisture. I have therefore added to the Sym- 

 piesometer a new Hygrometer, which is afterwards 

 described in p. 492. 



In some of the Sympiesometers which I have 

 made, the scale is divided into parts corresponding 

 to the increase in bulk which takes place in the gas, 

 by the diminished pressure of the atmosphere on as- 

 cending a given height, the temperature being 32° 

 of Fahrenheit. This scale is also formed by expe- 

 riment, as follows : The instrument being placed in 

 the glass-case as before described, increase the den- 

 sity of the inclosed air, until it support a column of 

 quicksilver of 31 inches, the temperature being 32°. 

 Mark this point zero ; then, from the logarithm of 

 31 substract .0100, and find the corresponding num- 

 ber, which is 30.294 ; regulate the density of the 

 air to support a column of quicksilver of this length ; 

 number this point on the scale 100, and divide the 

 space into 100 parts ; each part will equal the in- 

 crease of bulk or fall of the oil in the tube by as- 

 cending one fathom. In the above manner proceed, 

 by subtracting .0100 from the logarithm last found, 

 and marking the points corresponding to these den- 

 sities, until the scale is complete. 



By the above scale, the approximate height will 

 be given without the aid of a table of logarithms, 

 by subtracting the number of fathoms indicated by 



